The proposed study will investigate the nature of the ligand interaction with membrane IgM which is needed for B lymphocyte stimulation. This will be accomplished through the use of a panel of anti-human IgM monoclonal antibodies (MoAbs) which differ in affinity and/or epitope specificity. Using these defined anti-Ig ligands, an explanation for the diverse regulatory effects of anti-Ig ligands which have been observed in the past will be sought. These studies may elucidate the factors that determine whether or not autologous anti-Ig constant region antibodies (rheumatoid factors) can regulate the function of B cells to which they bind. The different mouse anti-human IgM MoAbs will be compared in their ability to a) induce resting B cells to proliferate, b) affect the proliferation and differentiation of B cells stimulated with other mitogens, and c) stimulate early changes in B cells which precede the S phase of the cell cycle. Once sets of stimulatory and non-stimulatory (perhaps inhibitory) MoAbs have been identified, explanations for the functional differences in these ligands will be sought. This will involve comparing stimulatory and non-stimulatory MoAbs and their F(ab')2 fragments for a) affinity and stoichiometry of binding to IgM and b) epitope specificity. The epitope specificity of the MoAbs will be investigated by competitive inhibition studies and by analysis of the ability of each MoAb to bind to chemically modified IgM, IgM fragments, and synthesized peptides which have been predicted by hydrophilicity and conformational analysis as likely to constitute immunogenic epitopes on the human IgM molecule. In addition, the nature of membrane molecule crosslinkage which is necessary to stimulate B cell proliferation will be studied. This will involve comparing the mitogenic capacity of a divalent anti-IgM MoAb to a hybrid antibody which consists of one F(ab') arm directed to IgM and one F(ab') arm directed to a human major histocompatibility (MHC) antigen.